The present invention relates to optical circuits and, more particularly, to flexible optical circuits having optical fibers encapsulated therein.
High capacity electronic systems are increasingly adopting optoelectronics as a means to surpass conventional limitations (e.g., transmission speed) of electrical interconnections. Although photonic technology has long been preferred in long-haul communications, optics is now quickly becoming a viable option for short link applications. One demanding short-link application for optical interconnection is in the interboard/shelf or backplane level of communication. Most large system equipment today is partitioned into bookshelf levels consisting of multiple printed wiring boards inserted into shelves within a frame or cabinet. One interconnect level within such a system is that between two printed circuit boards within the cabinet, known as the backplane level of interconnection.
Backplane systems are typically organized by mounting various system components on printed wiring boards and interconnecting the printed wiring boards with a circuit transmission element known as a backplane. The backplane may include various socket elements for receiving printed wiring boards. However, as the circuit density of printed wiring boards increases, it becomes difficult to provide the needed backplane interconnections because, as interconnection transmission lines become thinner, their impedances increase. Furthermore, electromagnetic interference between closely adjacent electrical signal parts can reduce signal integrity due to cross-talk and interference. Additionally, the distance over which information must be transmitted by backplane conductors is fairly long compared to the distances transmitted on printed wiring boards. These factors may reduce the speed at which the circuits can be operated, and the signal integrity, which may defeat a principal advantage of higher circuit densities. Optical fiber interconnections have been suggested to address these problems.
Recently, convenient and manufacturable methods of linking components at the backplane level via optical fiber interconnections have been suggested which may result in a number of advantages, including down-sized wiring closets, fewer cumbersome cables through management of connections, low loss distribution, and low cost. These optical fiber interconnections are often made of flexible material so that they can be bent for mounting in an appropriate structure to reduce the volume required by the system and to aid in connection to other electronic systems. Although advances in optical fiber interconnections will be discussed with focus on implementation at the backplane level, these interconnections can be utilized in a number of short link applications other than simply those used as backplane connections within a large system cabinet.
U.S. Pat. No. 5,259,051, to Burack et al. (hereinafter Burack et al. ""051), assigned to ATandT Bell Laboratories, the predecessor in interest of the assignee of the present invention, is incorporated herein by reference, and describes a method for making optical circuits for use as backplanes by using a robotic routing machine to apply optical fibers to a flat surface of a flexible plastic substrate. Optical fibers are bonded to the substrate surface by a pressure-sensitive adhesive, and after routing they are covered by a plastic sheet that encapsulates the fibers. The sheet is applied using lamination techniques, including the application of pressure and heat directly onto the plastic sheet. The purpose of the encapsulation is to give the structure mechanical stability, to protect the optical fibers from the surrounding environment, and to keep the optical fibers in place during the handling of the optical backplane. The optical fibers of the optical backplane are typically used as large-capacity transmission lines between printed wiring boards or between optical circuits. The optical backplane is preferably designed by a computer, which provides optical fiber routes of the appropriate length between input and output ports of the optical backplane. A robotic routing machine preferably implements these routes because it is important for optical transmission reliability that there not be undesirable deviations in the prescribed length of each line.
While the methods and apparatuses of the Burack et al. ""051 patent have been implemented with great success, it has been recognized that optical fibers, which are usually made of glass, are susceptible to damage, particularly at locations at which the fibers overlap or cross over one another. After the optical fibers have been encapsulated in plastic, strain on an optical fiber at a point of overlap or crossover may be sufficient to break or otherwise diminish performance of the optical fiber. The pressure applied to the plastic sheet which encapsulates the optical fibers may also break the optical fibers, especially at points where the optical fibers overlap or criss-cross each other, which is necessary for fiber routing.
U.S. Pat. No. 5,292,390, to Burack et al. (hereinafter Burack et al. ""390), incorporated herein by reference, is assigned to ATandT Bell Laboratories, the predecessor in interest of the assignee of the present invention, and also discloses optical fiber encapsulating techniques for producing optical circuits where a plurality of optical fibers are first bonded to an upper surface of a flat flexible plastic substrate and then are covered with a sheet of thermoplastic material to form a composite structure including the thermoplastic material, the optical fibers and the plastic substrate. This composite structure is then compressed at a first elevated temperature and at a first relatively high pressure which are sufficient to bond or tack the thermoplastic material to the plastic substrate. After cooling, a second elevated temperature is applied to the thermoplastic material while compressing the composite structure at a second pressure. The second elevated temperature is higher than the first temperature and is sufficiently high to cause the thermoplastic material to flow about and encase the optical fibers. Although this method of encapsulating fiber can result in less optical fiber breakage and a more reliable laminated structure than Burack et al. ""051, the fibers are still susceptible to damage. These problems occur particularly when an extremely dense array of optical fibers including many cross-overs is included on the surface of the flexible plastic substrate. Like Burack et al. ""051, the heat and pressure applied to the surface of the thermoplastic material may cause breaks in the optical fibers, especially at cross-over points. Furthermore, if damage from the process is sought to be avoided by using less pressure and lower temperatures, the stability and dependability of the encapsulation may sometimes be compromised.
A third method of encapsulating optical fibers in flexible optical circuits is discussed in U.S. Pat. No. 5,394,504, assigned to ATandT, which is incorporated herein by reference. This patent attempts to alleviate the problems with the previously mentioned patents by using a differential in air pressure, rather than mechanical means, to laminate the flexible optical circuit. Using a vacuum, a thermoplastic sheet is pressed against optical fibers residing on a flat member. The sheet is then heated to cause the thermoplastic sheet to adhere to the flat member, thereby encapsulating the optical fibers. While this application may improve upon earlier techniques of optical fiber encapsulation, there remain several unwanted consequences. For instance, because two solid plastic sheets are laminated together, air can get caught within the plastic sheets, resulting in air pockets. These air pockets may result in the lamination failing and the optical connections faltering. Furthermore, because pressure is applied as in the above examples, albeit from a vacuum, the optical fibers can break during encapsulation, especially at cross-over points. Finally, the use of a vacuum requires additional components may make the lamination process much more complicated and more expensive.
Accordingly, there is a continuing need for methods and apparatuses for encapsulating optical fibers on a flexible plastic substrate which may encase optical fibers, including optical fiber crossovers, with a reduced likelihood of damaging or weakening any of the fibers. Moreover, there is a need for an encapsulation method which is consistent with the requirements of mass production, which does not require great operator skill, which provides long-life structural support and protection to optical fibers, and which maintains predetermined fiber placement.
According to one embodiment of the invention, a flexible optical circuit having a plurality of optical fibers is provided by mounting optical fibers to a flexible porous substrate and thereafter encapsulating the optical fibers and porous substrate with a protective encapsulating material. The encapsulating material can be a liquid which surrounds the optical fibers and flows through pores in the porous substrate to achieve a structurally sound and protected flexible optical circuit. Because the encapsulating material surrounds and protects the optical fibers, heat and pressure are not required in forming the optical circuit, resulting in reduced optical fiber breakage as compared to prior art methods of constructing flexible optical circuits.
According to one aspect of the invention, an optical circuit includes a first flexible porous substrate having an upper surface and a lower surface, an adhesive located on the upper surface of the first flexible porous substrate, and optical fibers attached to the upper surface of the flexible porous substrate by the adhesive. The flexible optical circuit also includes an encapsulating material applied to the optical fibers. The encapsulating material completely surrounds the optical fibers and impregnates the first flexible porous substrate to provide strength and protection to the optical circuit.
The flexible optical circuit may be fabricated by providing a flexible porous substrate with an upper surface and a lower surface, adhering optical fibers to the upper surface of the flexible porous substrate, and applying an encapsulating material to the optical fibers and the flexible porous substrate to provide strength and protection to the optical circuit. Adhesive can be applied to the upper surface of the flexible porous substrate to cause the optical fiber to adhere to the upper surface of the flexible porous substrate. Furthermore, the flexible porous substrate can comprise a flexible mesh substrate made of kevlar, cotton, plastic, metal, cloth, or other suitable strong and flexible materials. The encapsulating material can be applied to the optical circuit by spraycoating the optical fibers and the flexible porous substrate or by dipping the optical circuit into a bath of encapsulating material. Upon curing, the encapsulating material should be flexible and durable. A suitable encapsulating material may be, for example, plastic.
According to another aspect of the invention, optical fibers are positioned between two porous substrates, and the optical fibers and both of the porous substrates are encapsulated by an encapsulating material. The circuit may be fabricated by providing a first flexible porous substrate with an upper surface and a lower surface, applying adhesive to the upper surface of the first flexible porous substrate or to the optical fibers to cause optical fibers to adhere to the upper surface of the first flexible porous substrate, placing the optical fibers on the upper surface of the first flexible porous substrate, placing a second flexible porous substrate on the optical fibers, located opposite the first flexible porous substrate, and applying an encapsulating material to the optical fibers, the first flexible porous substrate, and the second flexible porous substrate to provide strength and protection to the optical circuit.
Additionally, an adhesive may be applied to a surface of the second flexible porous substrate that is in contact with the optical fibers to fix the second flexible porous substrate relative to the optical fibers. The encapsulating material may be applied to the first and second flexible porous substrates by spray-coating or bath emersion so that the encapsulating material flows through pores in the second flexible porous substrate and pores in the first flexible porous substrate to fill any gaps therebetween, and to substantially surround the optical fibers therebetween. As in the previous described embodiments, the use of porous substrates and encapsulating material provide protection to the optical circuit without the need for lamination or the requirement of pressure. Therefore, optical fiber breakage and optical circuit delamination can be minimized.
Other features and advantages of the present invention will become apparent to one skilled in the art upon examination of the following drawings and detailed description. It is intended that all such features and advantages be included herein within the scope of the present invention as defined by the appended claims.